The difference between someone who has fantasies and someone who goes to jail for molesting kids is impulse control. It seems that was something he lacked when he was young. As he matured, he increasingly realized what he was doing was wrong and stopped the behavior. Fortunately, no child was hurt by his actions – at least as far as you know.
That all happened 15 or more years ago. He’s right to feel guilty about it. Guilt is one of the emotions that helps keep us all on the moral side of life. But it’s also important that he gives himself credit for having enough restraint that he stopped himself before it got even more serious.
There is no harm in being aroused by thinking about adult feet or by massaging the feet of an adult who consents. That’s what is called a benign paraphilia. It doesn’t hurt self or others. It’s an activity that is part of his private sexual life.
As far as interest in kids: What’s important now is whether he is being honest with himself and with you about his feelings and inclinations. If he is aroused by children or children’s feet, I hope he will talk with a therapist about how to manage those feelings so he doesn’t hurt a child or himself. If he truly isn’t sexually interested in children, he needs to find a way to forgive himself for his adolescent behavior and focus on living an upstanding adult life.
I can’t tell you whether or not you should be concerned. It all rests on your assessment of his ability to be honest with himself and with you.
I wish you both well.